In electronic engine control systems for, for example, twin gas turbine engines used in helicopters, it is desirable to control the free turbine speed isochronously at a fixed reference point, for example, at one hundred (100%) percent. Isochronously refers to there being no error between a reference signal and a feedback signal.
It is also desirable in such a system to have a more robust control system with increased bandwidth. Additionally, it is desirable to provide isochronous control, while expanding the transient bandwidth without associated integrator problems. These desirable attributes are achieved as part of the present invention.
It is also desirable to have superior control system performance in such a system. For example, normal speed governing typically involves a proportional-plus-integral (P+I) approach, when the free turbine speed (NF) loop is commanding fuel flow. When direct free turbine speed (NF) control is overridden by another control loop [e.g., acceleration (accel.), deceleration (decel.)]via the control loop selection logic, the NF error is operated upon by a proportional-plus-differential (P+D) approach. In the invention superior transient control is achieved through the use of a multi-loop speed governing design, as will be explained more fully below.
Additionally, there are a number of problems in the helicopter engine control art currently encountered in present control selection processes (e.g., overshoot/ring or early cut back). For example, a "stiff" rate limit will delay selection of the limit function, allowing an overshoot of the limiter, when used in conjunction with a well damped loop gain function to also reduce the system "ring" or "hard cut back" due to hitting the loop hard. In the invention a rate limit feature, added to the limit loop selection and control process, provides a method for control of the rate of approach to the limits built into the control system. The rate limit technique of the invention allows maximum use of available plant performance.
Thus, three aspects of the present invention meet these desires of the prior art or obviate these prior art problems particularly in the helicopter engine control art, as well as provide innovations applicable to engine control generally, whether single or multiple engine(s) systems and whether in helicopter or other applications, and even in some instances to the control of functions generally.
Additionally, it is noted that controls mounted in twin engine (e.g. gas turbine) helicopter applications generally require control of both output shaft speed and torque matching, presenting significant problems in terms of the failure mode effects of the torque input. A final aspect of the present invention provides a predetermined limit to the governed speed excursion in event of an in-range failure of the torque signal, or an unannounced fail fix of one engine or system.
With respect to this aspect of the invention, it is noted that current torque matching algorithms provide a path for significant speed excursions in the event of a failure, (e.g., incorrect bias, counter problems, "fail fix" on one engine, etc.). To limit this problem, the general, prior art practice has been to severely limit the torque matching gain, and therefore the effectiveness of the loop. The torque matching bias and the speed loop bias both contribute to the inner loop reference.
The torque matching problem arises when the error in the torque match loop cannot be closed (this problem can have many causes), and the difference causes a constant bias to the speed governing loop. In this case the speed loop is required to correct for this torque match error, causing a speed governing error proportional to the relative gains of the speed governing loop and the torque matching loop and opposite in sign. This problem is compounded in isochronous controls, as used in the exemplary electronic engine control design of the invention, because a steady state error can iterate to the extremes of the integrator limits.